Conventionally, various methods for performing tracking correction and crosstalk cancellation (hereinbelow, called “CTC”) are proposed in the field of an information recording/reproducing apparatus for an optical disc such as a CD (Compact Disc) and a DVD (Digital Versatile Disc). At present, a method of converting light emitted from a light source to three beams of a main beam (zeroth-order beam) and sub beams (± first-order beams) and performing tracking correction or CTC using the main beam and the sub beams is generally employed (for example, a differential push pull (DPP) method).
In the tracking correction method and the CTC method using the three beams, positions to be irradiated with the three beams are determined in advance in a track formed on the surface of a disk. When the positional relations are lost, the tracking correction or CTC cannot be performed properly. For example, in the DPP method, the relation that the phase of a push pull signal of the main beam and that of a push pull signal of the sub-beam are opposite to each other has to be maintained. It is consequently necessary to emit the sub-beams to positions shifted at a half track pitch (that is, on a land track) in the track normal direction. When the relation of the positions is lost, an accurate tracking error signal cannot be obtained.
On the other hand, the track pitch of an optical disk varies according to recording formats at present. In a so-called compatible recorder (an apparatus for recording/reproducing data to/from optical disks of different recording formats), various methods for satisfying the positional relations have been proposed in accordance with the kinds of optical disks to be recorded/reproduced. For example, Patent Document 1 discloses a method of converting a light beam emitted from a light source to five beams by a diffractive grating that generates not only a zeroth-order beam and ± first-order beams but also ± second-order beams and switching the light beam used for tracking correction in accordance with the kind of an optical disk to/from which data is recorded/reproduced.
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-5859